To characterize the structural and enzymatic composition of melanin granules throughout their development in vivo, both in normal and abnormal pigmentary systems. Our research has been aimed primarily at characterizing the chemical composition of melanosomes in the mouse, both in the normal black mouse as well as in melanoma tissue. Eyes are excised and homogenized in 0.25M sucrose, and the melanosomes are then isolated and purified by means of an extensive series of differential centrifugations. Granules thus isolated are solubilized in either sodium decyl sulfate, 8 M urea of Triton X-100, then examined with the aid of polyacrylamide gel electrophoresis. Computer analysis of the resulting protein separations allows the calculation of various physio-chemical parameters of these macromolecules, including molecular weight, molecular size, valence and several other physical constants that are most useful in the comparisons of various proteins.